4 Answers
4

The opposite/complement of digressing is sticking to the intended subject matter.

As for the action of getting back to that position after a digression, OP's "To get back on topic,...", or "To return to the subject,..." are perfectly normal. A possible one-word alternative might be...

Resume (“To start (something) again that has been stopped or paused from the point at which it was stopped or paused; continue, carry on”) and return occasionally are so used in phrases in fiction, like “After that digression he resumed...”, or a narrator might say, “To resume our story” or “To return to my point”. Recollect, in sense “To collect (things) together again”, and retrieve (“To remember or recall something”) also are used: “Recollecting himself, he continued:” or “Retrieving his thoughts, he went on”. All sorts of other phrases (but few single, unsupported verbs) are used: “But where was I?”, “Less digressively”, “More topically”, “But I digress”, “Resuming our thread”, “That aside”.

Regress could work, but if you digress, then it's closer to deviate in its use in this sense.

So I don't think there is a single word for un-digress in this context.

In the sentence "let me just digress and speak about XYZ," you would use the sentence "anyway, to return to the main topic..." so I think "return" would work best as you are returning from a digression.